Hi there, Emily!
Welcome to the Disney Parks Moms Panel!  Your 
Disney Cruise Line vacation is just days away, can you believe it?!?!  I am so excited for you and know you must be eagerly counting down the days (maybe even minutes!) until you step foot onboard your home at sea for a vacation you won't soon forget!
Castaway Cay is one of my favorite places to visit when sailing with Captain Mickey!  I love spending a relaxing day at the beach with my family AND my favorite Disney Characters!  I also love that you can break away from the relaxing beach and explore the island – either during the Castaway Cay 5K (register for this at Guest Services once onboard, if interested) or with an exciting Port Adventure!  While I have not done the 
jet ski adventure at Castaway Cay yet (because my boys aren’t quite big enough yet), I have visited with many Guests that have – and they had nothing but great things to say about their experience!  Guests are permitted to bring a GoPro to document their exciting adventure (at your own risk, of course!) and from my resEARch, it looks like you will need to bring your own mount – and either a chest harness or wrist mount would work!  I found that the majority of Guests who have shared their experiences online used a chest harness, but there were some that utilized a wrist strap with success, too!  So, I would recommend doing what is most comfortable for you!  
After your exciting adventure on the water, be sure to recharge with a delicious lunch at Cookie’s BBQ and don’t forget to grab an extra-large chocolate chip cookie!  You may even want to grab two and sandwich them around some soft serve ice cream for a yummy lunchtime dessert!
Wishing you an exciting adventure at sea, Emily!  Say hi to Captain Mickey for me!
Happy Sailing!
Stephanie